1. Field of the Invention
The present invention relates to power amplifiers, particularly switched-mode power amplifiers.
2. Description of Related Art
Switched-mode power amplifiers have demonstrated the capability of producing, with high power-added efficiency (PAE), phase-modulated signals that have very high signal qualityxe2x80x94i.e., low root-mean-square (RMS) phase error relative to an ideal signal and little or no degradation in power spectral density (PSD). These power amplifiers have also been demonstrated to be highly tolerant of temperature variation, and are believed to be highly tolerant to fabrication-process variation, making them attractive for high-volume applications such as consumer electronics. Such power amplifiers include a switch connected to a resonant network; the output of the resonant network is connected in turn to a load (e.g., the antenna in a radio transmitter).
An early switched-mode amplifier is described in U.S. Pat. No. 3,900,823 to Sokal et al., incorporated herein by reference. Sokal et al. describes the problem (created by unavoidable feedthrough from amplifier input to amplifier output) of power control at low power levels and proposes solving the problem by controlling RF input drive magnitude to a final amplifier stage. In particular, the input drive magnitude of the final stage is controlled by using negative feedback techniques to control the DC power supply of one or more stages preceding the final stage. Various other known techniques use variation of amplifier power supply for linearization as described, for example, in the following patents, incorporated herein by reference: U.S. Pat. Nos. 5,091,919; 5,142,240, and 5,745,526.
Another type of switched mode amplifier, that does not require the use of negative feedback as in Sokal, is described in U.S. patent application Ser. Nos. 09/247,095 and 09/247,097 of the present assignee, entitled HIGH-EFFICIENCY MODULATING RF AMPLIFIER and HIGH-EFFICIENCY AMPLIFIER OUTPUT LEVEL AND BURST CONTROL, respectively, filed Feb. 9, 1999 and U.S. patent application Ser. No. 09/637,269, entitled HIGH-EFFICIENCY MODULATING RF AMPLIFIER, filed Aug. 10, 2000, all incorporated herein by reference. In the latter switched-mode power amplifiers, the average power is determined by two signals: the switch supply signal and the switch control signal. The switch supply signal is the DC voltage available on one side of the switch; as this voltage increases, the peak voltage of the oscillatory signals developed within the resonant network and subsequently delivered to the load also increases. The switch control signal is typically a phase-modulated signal that controls the switch (i.e., determines whether the switch is on or off). This switch control signal should be strong enough to toggle the switch on and off but should not be excessively strong: unlike a linear amplifier in which the strength of the output signal is determined by the strength of the input signal, in a switched-mode power amplifier, if the switch control signal is too strong, the excess signal merely leaks through the switch and into the resonant network (i.e., feedthrough). When this occurs, a version of the switch control signal that is out-of-phase with respect to the desired signal adds to the desired signal within the resonant network, altering both the phase and the amplitude of the output signal in an undesirable way.
French Patent 2,768,574 also describes a switched-mode power amplifier arrangement. Referring to FIG. 1, in this arrangement, the power amplifier circuit comprises a DC-to-DC converter 20 and a power amplifier 30. The DC-to-DC converter 20 includes a pulse-width modulator 22, a commutator/rectifier 24 and a filter 26.
The pulse-width modulator 22 is coupled to receive a DC-to-DC command input signal from a signal input terminal 21, and is arranged to apply a pulse-width-modulated signal to the commutator/rectifier 24. The commutator/rectifier 24 is coupled to receive a DC-to-DC power supply input signal from a signal input terminal 25, and is also coupled to apply a switched signal to filter 26. The filter 26 in turn applies a filtered switched signal 28 in common to multiple stages of the power amplifier 30.
A circuit of the foregoing type is substantially limited by the frequency of the pulse-width modulator. In addition, common control of multiple power amplifier stages in the manner described may prove disadvantageous as described more fully hereinafter.
It is desirable to achieve more precise control of switched-mode-generated RF signals, including amplitude-modulated signals, such that the aforementioned benefits of switched-mode power amplifiers may be more fully realized.
This invention controls and modulates switched-mode power amplifiers to enable the production of signals that include amplitude modulation (and possibly, but not necessarily, phase modulation), the average power of which may be controlled over a potentially wide range.
In order to produce amplitude-modulated signals, the DC switch supply voltage is replaced by a time-varying switch supply signal that is related to the desired amplitude modulation. This switch supply signal can be either the desired amplitude modulation signal itself or a pre-distorted version thereof, where the pre-distortion is such that the output signal has the desired amplitude modulation. In the latter case, the pre-distortion corrects for amplitude non-linearity (so-called AM/AM distortion) in the switch and/or the resonant network.
The foregoing modification alone, however, may be insufficient to provide as much dynamic range in the output signal as may be desired. Also, the modification may not be sufficient to maintain dynamic range in the amplitude modulation while adjusting the average power of the output signal. Both of these problems are caused by the undesirable leakage signal described previously; its contribution to the output is largely independent of the level of the switch supply signal. That is, the switch supply signal may be reduced to zero volts (the minimum possible amplitude), yet the output signal will still be at a relatively high level; below some point, the amplitude modulation imparted through the switch supply signal is manifest less and less in the output signal.
Similarly, the severity of amplitude-dependent phase shift (so-called AM/PM distortion) increases as the switch supply signal decreases. This effect arises because the leakage of the switch control signal is out of phase relative to the desired signal. As the switch supply signal decreases, the desired signal decreases as well, whereas the leakage signal does not; since these two signals are out of phase, the phase of their sum is increasingly dominated by the phase of the leakage signal. This invention, in one aspect thereof, modifies the switched-mode power amplifier by adjusting the amplitude of the switch control signal to reduce the undesirable leakage effect. As a result, it becomes possible to produce output signals having average power anywhere within a wide range, or to greatly increase the dynamic range over which amplitude modulation may be produced at a given average power level, or both.